This application relates to an apparatus and method for providing patterned illumination fields for use within process control and article inspection applications. More specifically, it pertains to the use of patterned illuminators to enable visual surface inspection of polished objects such as ball bearings. While the invention is particularly directed to the art of high-speed automated process control and article inspection applications, and will be thus described with particular reference thereto, it will be appreciated that the invention may have usefulness in other fields and applications. For example, the invention may be used as an illumination source for manual or operator-initiated inspection of parts.
By way of background, the use of engineered lighting systems within automated inspection systems is well known in the art. For example, U.S. Pat. No. 4,882,498 entitled “Pulsed-Array Video Inspection Lighting System” describes a method and apparatus related to engineered lighting system for machine vision applications. Amongst other things this invention introduces the concepts of spatially distributing and high-current pulsing solid-state lighting elements within the engineered illumination system. Both of these innovations worked to allow autonomous machine vision to be advantageously applied to an expanding class of application areas. Similarly, U.S. Pat. No. 6,238,060 entitled “Machine Vision Ring-Reflector Illumination System and Method” describes an innovation related to the implementation of a ringlight-based illumination system. One embodiment of this referenced patent is an illumination system capable of generating configurable dark field illumination. There is a volume of prior art descriptions which indicates the appropriateness and usefulness of dark field illumination to highlight surface anomalies (such as scratches and pits) occurring on generally flat polished surfaces. While introducing improvements in the areas of ease of deployment and dark field configurability, the referenced invention is limited in generally the same fashion as all prior art ringlight implementations in regards to its ability to provide complete and uniform illumination fields for the case of contoured, 3-dimensional, specular surfaces. On these classes of parts, the directional lighting associated with ringlight illumination causes uneven returns from the part under inspection. In addition to getting scatter-induced reflections at defect locations such as scratches and pits (which are preferred), intense image artifacts are also generated at all locations wherein the local surface orientation (due to normal part contours) allow light emitted by the ringlight source to be specularly-reflected into the receiver aperture of an associated inspection system. As these image artifacts or hot-spots are typically intense enough so as to drive any receiving entity into high scale saturation, inspection (automated or otherwise) is not generally possible in these areas. In addition, due to general shadowing effects caused by the 3-dimensional nature of the part under inspection, it is possible for areas of the part to receive little to no incident illumination if ringlight or other dark field illumination technique is exclusively used to image these classes of parts.
Many of the objects typically inspected using automated machine vision systems happen to be specular and 3-dimensional in nature. Included in this list are metal cans and closures, ball bearings, electrical contacts, and populated electrical assemblies. To adequately inspect these and other complex parts, hundreds of additional prior art improvements to the illumination portions of these inspection systems have been devised and implemented. To address the specific inspection needs associated with highly specular, 3-dimensional parts, various forms of an illumination system typically described as a continuous, diffuse illuminator have been designed, constructed, and applied within inspection systems. They are well known in the art. Such a continuous diffuse system was described in the 1985 textbook, Automated Visual Inspection, by Batchelor, Hill, and Hodgson. These lighting systems differ from directional ring light sources in that they are designed to generate lighting fields that illuminate parts under inspection from essentially all directions. These illuminators often take the geometric form of a hemisphere surrounding the part under inspection. Constructed in this manner, continuous, diffuse lighting systems allow uniform gray scale images of highly specular, 3-dimensional parts to be acquired. Images that are generally uniform in gray scale intensity advantageously enable automated, high-speed analysis. When illuminated in this fashion, specular, 3-dimensional parts can be adequately inspected for a number of classes of defects including absorptive-type defects (such as contamination) as well as gross part deformations.
While it is sometimes possible to detect surface scratches or pits using continuous diffuse lighting systems, the performance of these systems on these classes of defects is not optimal. The angularly-uniform and complete nature of the impinging light stimulus creates a condition wherein scatter effects caused by surface scratches and the like are effectively blanketed over and masked by natural specular reflections occurring on the surrounding non-defective surface. To best highlight these types of defects on highly specular parts, it is still best to use directional lighting.
As an illustration of one specific example of a continuous, diffuse lighting system, U.S. Pat. No. 6,341,878 entitled “Method and Apparatus for Providing Uniform Diffuse Illumination to a Surface” describes an improvement in the state of the art of uniform diffuse illumination which places the emitting light source below the part under inspection. In doing so, the ability of the lighting system to generate low angle illumination is improved. This invention also references a hemispherical diffuser located above the part under inspection and the deployment of crossed linear polarizers (one over the emitting light elements and one at the receiver aperture).
The present invention overcomes difficulties noted above and others. It also provides distinct advantages over heretofore known systems.